Method and apparatus for image encoding/decoding

ABSTRACT

Disclosed are a method and an apparatus for image encoding/decoding. The method for image decoding includes comparing whether a size of a current transform block and a size of a smallest transform block (smallest transform unit (STU)) are the same as each other, and decoding information indicating whether transformation is performed on the current transform block and determining whether to perform inverse transformation on the current transform block based on the decoded information indicating whether the transformation is performed when the size of the current transform block and the size of a smallest transform block are the same as each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/990,608 filed on Jan. 7, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/191,976 filed on Feb. 27, 2014 and issued asU.S. Pat. No. 9,247,255, which claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2013-0022145 filed on Feb.28, 2013 in the Korean Intellectual Property Office, and Korean PatentApplication No. 10-2013-0024978 filed on Mar. 8, 2013 in the KoreanIntellectual Property Office, the entire disclosures of which areincorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Field of the invention

The present invention relates to image encoding/decoding, and moreparticularly, to a method and an apparatus for encoding/decoding animage by determining whether to transform the image.

Related Art

In recent years, a demand for high-resolution and high-quality imagessuch as a high definition (HD) image and an ultra high definition (UHD)image has increased in various application fields. As image data hashigh resolution and high quality, data quantities are relativelyincreased as compared with the existing image data, and as a result,when image is transmitted by using a medium such as the existingwired/wireless wideband line or the image data is stored by using theexisting storage medium, transmission cost and storage cost areincreased. In order to solve the problems which occur as the image datahas high resolution and high quality, high-efficiency image compressiontechnologies may be used.

The image compression technologies include various technologiesincluding an inter prediction technology of predicting a pixel valueincluded in a current picture from a picture before or after the currentpicture, an intra prediction technology of predicting the pixel valueincluded in the current picture by using pixel information in thecurrent picture, an entropy coding technology of allocating a shortsymbol of which an appearance frequency is high and allocating a longsymbol of which the appearance frequency is low, and the like, and theimage data is effectively compressed to be transmitted or stored byusing the image compression technology.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and anapparatus for image encoding/decoding that can improve encoding/decodingefficiency. Another object of the present invention is to provide amethod and an apparatus that determine whether to transform an imagebased on a size of a smallest transformation block capableencoding/decoding efficiency.

In accordance with an embodiment of the present invention, there isprovided a method for image decoding. The method includes comparingwhether a size of a current transform block and a size of a smallesttransform block (smallest transform unit (STU)) are the same as eachother, and decoding information indicating whether transformation isperformed on the current transform block and determining whether toperform inverse transformation on the current transform block based onthe decoded information indicating whether the transformation isperformed when the size of the current transform block and the size ofthe smallest transform block are the same as each other, the smallesttransform block is a transform block having the minimum size determinedby an encoder.

In the determining of whether to perform the inverse transformation onthe current transform block, when the information indicating whether thetransformation is performed indicates that the transformation is notperformed, the inverse transformation of the current transform block maybe skipped.

In the determining of whether to perform the inverse transformation onthe current transform block, when the information indicating whether thetransformation is performed indicates that the transformation isperformed, the inverse transformation of the current transform block maybe performed.

The size of the smallest transform block may be larger than a size of4×4.

The method may further include decoding information on a flag indicatingwhether to use a function to determine whether to perform inversetransformation on the current transform block based on the size of thesmallest transform block, whether to perform the inverse transformationon the current transform block may be determined by referring to thesize of the smallest transform block when the flag information indicatesthat the function to determine whether to perform the inversetransformation on the current transform block based on the size of thesmallest transform block is used, and whether to perform the inversetransformation on the current transform block having the size of 4×4 maybe determined when the flag information indicates that the function todetermine whether to perform the inverse transformation on the currenttransform block based on the size of the smallest transform block is notused.

The method may further include performing inverse transformation on thecurrent transform block when the size of the current transform block andthe size of the smallest transform block are not the same as each other.

In accordance with another embodiment of the present invention, there isprovided an apparatus for image decoding. The apparatus include anentropy decoding module entropy-decoding information indicating a sizeof a smallest transform block and whether transformation is performed,and an inverse transformation module comparing whether the size of thesmallest transform block and a size of a current transform block are thesame as each other and determining whether to perform inversetransformation on the current transform block based on the informationindicating whether the transformation is performed when the size of thesmallest transform block and the size of the current transform block arethe same as each other, the smallest transform block is a transformblock having the minimum size determined by an encoder.

In accordance with yet another embodiment of the present invention,there is provided a method for image encoding. The method includecomparing whether a size of a current transform block and a size of asmallest transform block (smallest transform unit (STU)) are the same aseach other, and determining whether to perform transformation on thecurrent transform block when the size of the current transform block andthe size of the smallest transform block are the same as each other, thesmallest transform block is a transform block having the minimum sizedetermined by an encoder.

In the determining of whether to perform the transformation on thecurrent transform block, information indicating whether thetransformation is performed on the current transform block may beencoded according to the determination.

The size of the smallest transform block may be larger than a size of4×4.

In the determining of whether to perform the transformation on thecurrent transform block, when it is determined that the transformationof the current transform block is not performed, the information mayindicate that the transformation of the current transform block isskipped.

In the determining of whether to perform the transform on the currenttransform block, when it is determined that the transformation of thecurrent transform block is performed, the information may indicate thatthe transformation of the current transform block is performed and thetransformation of the current transform block is performed.

The method may further include performing transformation on the currenttransform block when the size of the current transform block and thesize of the smallest transform block are not the same as each other.

The method may further include encoding information on a flag indicatingwhether to use a function to determine whether to perform transformationon the current transform block based on the size of the smallesttransform block, whether to perform the transformation on the currenttransform block may be determined by referring to the size of thesmallest transform block when the flag information indicates that thefunction to determine whether to perform the transformation on thecurrent transform block based on the size of the smallest transformblock is used, and whether to perform the transformation on the currenttransform block having the size of 4×4 may be determined when the flaginformation indicates that the function to determine whether to performthe transformation on the current transform block based on the size ofthe smallest transform block is not used.

In accordance with still another embodiment of the present invention,there is provided an apparatus for image encoding. The apparatusincludes a transformation module comparing whether a size of a currenttransform block and a size of a smallest transform block are the same aseach other, and determining whether to perform transformation on thecurrent transform block when the size of the current transform block andthe size of the smallest transform block are the same as each other, andan entropy encoding module entropy-encoding information indicatingwhether transformation is performed on the current transform blockaccording to the determination of the transformation module, thesmallest transform block is a transform block having the minimum sizedetermined by an encoder.

Whether to transform a current transformation block is determined basedon a size of the smallest transformation block and the determinedinformation is efficiently signaled to increase encoding efficiency andimprove a subjective image quality.

If degree of spatial change of pixel values in a corresponding block tobe compressed in the image is very large or very sharp, and as a result,degree in which image energy concentrates on low frequencies in spite oftransformation is not significant and when serious image qualitydeterioration is caused by primarily maintaining low-frequencycomponents and removing high-frequency components or strongly applyingquantization during a compression process, whether to performtransformation may be determined based on the size of the smallesttransformation block without large overhead by judgment of an encoder toincrease image compression rate and minimize image qualitydeterioration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of an apparatusfor image encoding according to an embodiment of the present invention;

FIG. 2 is a block diagram illustrating a configuration of an apparatusfor image encoding according to an embodiment of the present invention;

FIG. 3 is a flowchart schematically illustrating a method for encodingan image by determining whether to perform transformation based on asize of a smallest transformation block according to an embodiment ofthe present invention; and

FIG. 4 is a flowchart schematically illustrating a method for decodingan image by determining whether transformation is performed based on asize of a smallest transformation block according to an embodiment ofthe present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. In describing the embodiments ofthe present specification, when it is determined that the detaileddescription of the known art related to the present invention mayobscure the gist of the present invention, the corresponding descriptionthereof may be omitted.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. Moreover, acontent of describing “including” a specific component in thespecification does not exclude a component other than the correspondingcomponent and means that an additional component may be included in theembodiments of the present invention or the scope of the technicalspirit of the present invention.

Terms such first, second, and the like may be used to describe variouscomponents, but the components are not limited by the terms. The aboveterms are used only to discriminate one component from the othercomponent. For example, without departing from the scope of the presentinvention, a first component may be referred to as a second component,and similarly, a second component may be referred to as a firstcomponent.

Further, components described in the embodiments of the presentinvention are independently illustrated in order to show differentcharacteristic functions and each component is not constituted byseparated hardware or one software constituting unit.

That is, each component includes respective components which arearranged for easy description and at least two components of therespective components may constitute one component or one component isdivided into a plurality of components which may perform theirfunctions. Even an integrated embodiment and separated embodiments ofeach component is also included in the scope of the present inventionwithout departing from the spirit of the present invention.

FIG. 1 is a block diagram illustrating a configuration of an apparatusfor image encoding according to an embodiment of the present invention.

Referring to FIG. 1, an image encoding apparatus 100 includes an interprediction module 110, an intra prediction module 120, a switch 125, asubtractor 130, a transform module 135, a quantization module 140, anentropy encoding module 150, a dequantization module 160, an inversetransform module 170, an adder 175, a filter module 180, and a picturebuffer 190.

The image encoding apparatus 100 may encode an input image in an intramode or an inter mode and output a bitstream. In the case of the intramode, the switch 125 is switched to ‘intra’, and in the case of theinter mode, the switch 125 is switched to ‘inter’. The image encodingapparatus 100 may generate a prediction block for an input block of theinput image and thereafter, encode a residual between the input blockand the prediction block.

In the case of the intra mode, the intra prediction module 120 mayperform a spatial prediction by using a pixel value of a pre-encodedblock around a current block to generate the prediction block.

In the case of the inter mode, the inter prediction module 110 mayacquire a motion vector by finding an area corresponding to the inputblock in a reference image stored in the picture buffer 190 during amotion estimation process. The inter prediction module 110 may perform amotion compensation by using the motion vector and the reference imagestored in the picture buffer 190 to generate the prediction block.

The subtractor 130 may generate a residual block by a residual betweenthe input block and the generated prediction block. The transform module135 may transform the residual block to output a transform coefficient.And the quantization module 140 may quantize the input transformcoefficient according to a quantization parameter to output a quantizedcoefficient.

The entropy encoding module 150 may perform entropy encoding based onvalues calculated by the quantization module 140 or an encoded parametervalue calculated during encoding to output the bitstream.

Since the image encoding apparatus 100 performs inter predictionencoding, the current encoded image needs to be decoded and stored foruse as the reference image. Accordingly, the quantized coefficient maybe inversely quantized in the dequantization module 160 and inverselytransformed in the inverse transform module 170. The inversely quantizedand inversely transformed coefficient is added to the prediction blockby the adder 175, whereby a reconstructed block is generated.

The reconstructed block passes though the filter module 180, and thefilter module 180 may apply at least one or more of a deblocking filter,a sample adaptive offset (SAO), and an adaptive loop filter (ALF) to thereconstructed block or a reconstructed picture. The reconstructed blockwhich passes through the filter module 180 may be stored in the picturebuffer 190.

FIG. 2 is a block diagram illustrating a configuration of an apparatusfor image decoding according to an embodiment of the present invention.

Referring to FIG. 2, the image decoding apparatus 200 includes anentropy decoding module 210, a dequantization module 220, an inversetransform 230, an intra prediction module 240, an inter predictionmodule 250, a filter module 260, and a picture buffer 270.

The image decoding apparatus 200 may receive a bitstream output from theencoding apparatus, perform decoding the bitstream in an intra mode oran inter mode and output a reconfigured image, that is, a reconstructedimage. In the case of the intra mode, the switch may be switched to‘intra’, and in the case of the inter mode, the switch may be switchedto ‘inter’.

The image decoding apparatus 200 may acquire a reconstructed residualblock from the received bitstream, and generate a prediction block andthereafter, a block reconfigured by adding the reconstructed residualblock and the prediction block, that is, a reconstructed block.

The entropy decoding module 210 may entropy-decode the receivedbitstream according to a probability distribution. A quantized(transformed) coefficient may be generated by the entropy decoding.

The quantized coefficient is inversely quantized in the dequantizationmodule 220 and inversely transformed in the inverse transform module230, and as a result of dequantization/inverse-transformation of thequantized coefficient, the reconstructed residual block may begenerated.

In the case of the intra mode, the intra prediction module 240 mayperform a spatial prediction by using a pixel value of a pre-encodedblock around a current block to generate the prediction block.

In the case of the inter mode, the inter prediction module 250 mayperform motion compensation by using a motion vector and a referenceimage stored in the picture buffer 270 to generate the prediction block.

The reconstructed residual block and the prediction block may be addedthrough an adder 255, and the added blocks may pass through the filtermodule 260. The filter module 260 may apply at least one of a deblockingfilter, an SAO, and an ALF to the reconstructed block or thereconstructed picture. The filter module 260 may output a reconfiguredimage, that is, a reconstructed image. The reconstructed image may bestored in the picture buffer 270 to be used in an inter prediction.

Hereinafter, the block may mean a unit of image encoding and decoding.Therefore, in the specification, the block may mean a coding unit (CU),a prediction unit (PU), a transform unit (TU), and the like in somecases. In the specification, an encoding/decoding target block may beused as a meaning including both transform/inverse transform targetblock when transform/inverse transform is performed and a predictiontarget block when a prediction is performed.

Meanwhile, in the image compression technology in the related art, aresidual signal corresponding to a difference value between an originalblock and the prediction block is always transformed and encoded.However, when degree of spatial change of pixel values in acorresponding block to be compressed is very large or very sharp, adegree in which image energy concentrates on low frequencies is notlarge in spite of performing the transform and significant image qualitydeterioration may be caused when low-frequency components are primarilymaintained and high-frequency components are removed or quantization isstrongly applied during a compression process. In particular, aprobability that such a problem will occur is higher in the case of anartificial image made by a computer. In this case, when the residual isimmediately encoded in a spatial area without transformation dependingon a characteristics of the image rather than indiscriminatelytransforming the residual signal like the existing method, compressionperformance may be improved or image quality deterioration may beremarkably reduced.

In order to solve such a problem, in high efficiency video coding (HEVC)technology which is being standardized at present, when the size of atransform block (TB) or a transform unit (TU) is smallest, that is, thesize of the transform block is 4×4, encoding may be performed without atransformation process by determining whether to transform a transformblock having a size of 4×4. Accordingly, a transformation process shallnot be skipped in a transform block having a larger size than 4×4 andonly in the case of the size of 4×4, the transformation process may beselectively skipped.

Meanwhile, in the present image compression technology, a minimum sizeof a transform block may be determined independently for each encoder.For example, in a specific application, a minimum size of a transformblock may be determined as not 4×4 but 8×8 or 16×16. When the minimumsize of a transform block determined by the encoder is 8×8 or more,transformation needs to be always performed in the present HEVCtechnology. Accordingly, when the present HEVC technology is applied,the transformation process is always performed even though it isadvantageous that the transformation process is skipped becausetransform efficacy of the block in the image deteriorates, a problem inwhich the compression rate decreases or the image quality deteriorationis caused occurs.

In order to solve the problems, the present invention provides a methodthat the transformation process may be skipped by determining whether toperform the transformation even though the minimum size of the transformblock determined by the encoder is variable. Hereinafter, thespecification, a transform block having the minimum size determined bythe encoder is referred to as a smallest transform block (STU).

FIG. 3 is a flowchart schematically illustrating a method for encodingan image by determining whether to transform based on a size of asmallest transform block according to an embodiment of the presentinvention. Respective steps of FIG. 3 may be performed within theconfiguration corresponding to the image encoding apparatus described inFIG. 1.

The encoder may perform the intra prediction or inter prediction of apresent encoding target block to generate the prediction block andthereafter, acquire a residual signal corresponding to a differencevalue between the present encoding target block and the predictionblock. In this case, the encoder may perform a process of transformingthe residual signal based on the transform block unit and thetransformation process may be performed by the method illustrated inFIG. 3.

Referring to FIG. 3, the encoder may compare a current transform blocksize and a size of a smallest transform block (S310). That is, theencoder may determine whether the current transform block size and thesize of a smallest transform block are the same as each other.

The size of a smallest transform block (STU) as the transform blockhaving the minimum size determined by the encoder (alternatively, auser) represents a transform block size in which the transformationprocess may be skipped. For example, the size of a smallest transformblock may be a size of 8×8, 16×16, 32×32, or the like larger than 4×4which is the smallest size pre-determined in the present HEVC. Ofcourse, the size of a smallest transform block may be the size of 4×4.

For example, the size of a smallest transform block may be set by using“log2_min_transform_block_size_minus2” which is information indicating aminimum size of transform block in the present HEVC. A syntax element of“log2_min_transform_block_size_minus2” may be set in a sequenceparameter set (SPS), a picture parameter set (PPS), or a slice header tobe transmitted to a decoder. Here, the size of a smallest transformblock N_min may be deduced by Equation 1 below by using“log2_min_transform_block_size_minus2”.N _(min)=2^((log2) ^(_) ^(min) ^(_) ^(transform) ^(_) ^(block) ^(_)^(size) ^(_) ^(minus2+2))   [Equation 1]

The size of a smallest transform block (that is, STU) may beN_(min)×N_(min) by Equation 1 above.

The size N of the current transform block may be deduced by Equation 2below by using “log2TrafoSize” which is a variable for indicating thesize of the current transform block.N=2^((log2TrafoSize))   [Equation 2]

The current transform block (that is, TU) size may be N×N by Equation 2above.

When the current transform block size and the size of a smallesttransform block are the same as each other, the encoder may determinewhether to transform the current transform block (S320). Here, a flagmay be used in order to notify whether transform is performed on thecurrent transform block. For example, transform_skip_flag which is theflag indicating whether transform is performed on the current transformblock may be used. When a value of transform_skip_flag is 1, it may beindicated that transform is not performed but skipped on the currenttransform block, and when the value of transform_skip_flag is 0, it maybe indicated that transform is performed on the current transform block.By determining whether to perform the transformation, when the currenttransform block size and the size of a smallest transform block are thesame as each other, the flag (for example, transform_skip_flag)indicating whether the transformation is performed is encoded to betransmitted to the decoder. Here, when the value of the flag (forexample, transform_skip_flag) indicating whether the transformation isperformed is 1 (S330), the encoder may skip transforming the currenttransform block (S340). If not so, the encoder may transform the currenttransform block (S350).

Meanwhile, as the judgment result of step S310, when the currenttransform block size and the size of a smallest transform block are notthe same as each other, the encoder may transform the current block(S350). In this case, the flag (for example, transform_skip_flag)indicating whether the transformation is performed is not encoded andthe flag information is not transmitted to the encoder.

FIG. 4 is a flowchart schematically illustrating a method for decodingan image by determining whether transform is performed based on a sizeof a smallest transform block according to an embodiment of the presentinvention. Respective steps of FIG. 4 may be performed within theconfiguration corresponding to the image decoding apparatus described inFIG. 2.

Referring to FIG. 4, the decoder may compare a current transform blocksize and a size of a smallest transform block (S410). That is, thedecoder may decode information on the size of a smallest transform blockand determine whether the current transform block size and the size of asmallest transform block are the same as each other. For example, whenthe information on the size of a smallest transform block is stored in“log2_min_transform_block_size_minus2”, the information of“log2_min_transform_block_size_minus2” may be decoded and thereafter,the size of a smallest transform blocklog2_min_transform_block_size_minus2+2 and the current transform blocksize log2TrafoSize may be compared with each other.

When the current transform block size and the size of a smallesttransform block are the same as each other, the decoder may decode aflag (for example, transform_skip_flag) indicating whether thetransformation is performed (S420).

The decoder may perform an inverse transformation or skip an inversetransformation process according to the decoded flag (for example,transform_skip_flag) information (S430).

For example, when the decoded flag indicates that the transformationprocess is skipped (for example, when a value of transform_skip_flag is1), the decoder does not perform the inverse transformation. When thedecoded flag indicates that the transformation is performed (forexample, when the value of transform_skip_flag is 0), the decoderperforms the inverse transformation.

Meanwhile, when the current transform block size and the size of asmallest transform block are not the same as each other, the decoder mayperform the inverse transformation on the current transform blockwithout decoding the flag (for example, transform_skip_flag) indicatingwhether the transformation is performed. The present invention mayprovide a method that indicates whether to use a function to skip thetransformation process based on a size of a smallest transform block asdescribed in FIG. 3 and a function to skip the inverse transformationprocess based on a size of a smallest transform block as described inFIG. 4. For example, the encoder may use a flag that indicates whetherto use the function to skip the transformation process based on a sizeof a smallest transform block. Since as a decoding process, an accurateinverse process to the encoding process needs to be performed, when theflag indicating whether to use the function to skip the transformationprocess based on a size of a smallest transform block during theencoding process indicates that the function is used, the flagrepresents that the function to skip the inverse transformation processis used based on a size of a smallest transform block even during thedecoding process similarly. The flag information is set in the sequenceparameter set (SPS), the picture parameter set (PPS), or the sliceheader to be transmitted to the decoder.

Table 1 below schematically illustrates one example of a case in whichthe encoder stores the flag information in the sequence parameter setand transmits the stored flag information to the decoder.

TABLE 1 Sequence_parameter_Set { ... transform_skip_enabled_flag    if(transform_skip_enabled_flag)       min_transform_block_size_based_transform_skip_flag ... }

Herein, min_transform_block_size_based_transform_skip_flag is a flagthat indicates whether to use the function to skip the transformationprocess based on a size of a smallest transform block during theencoding process.

When a value of min_transform_block_size_based_transform_skip_flag is 1,the flag indicates that the decoder determines whether to perform theinverse transformation by referring to a size of a smallest transformblock determined by the encoder, instead of referring to the smallestsize of 4×4 pre-determined in the present HEVC at the time ofdetermining whether to perform the inverse transformation. For example,when the encoder determines the size of 8×8 as a size of a smallesttransform block, the decoder determines whether to perform the inversetransformation, thereby skipping the inverse transformation process whenthe transform block size is 8×8. When the value ofmin_transform_block_size_based_transform_skip_flag is 0, the flagindicates that the decoder determines whether to perform the inversetransformation by referring to the smallest size of 4×4 pre-determinedin the present HEVC at the time of determining whether to perform theinverse transformation.

In implementing the decoding apparatus or the decoding method accordingto the present invention, the size of a smallest transform block to becompared with the current transform block size is calculated by anequation by referring to the value ofmin_transform_block_size_based_transform_skip_flag to implement thedecoding apparatus or method for a simpler configuration.

That is, min_transform_block_size_based_transform_skip_flag indicatesthat the function to skip the inverse transformation process is usedbased on the size of a smallest transform block (for example, when thevalue of min_transform_block_size_based_transform_skip_flag is set to1), log2_skipTrafoSize_min indicating the size of a smallest transformblock referred to at the time of determining whether to skip the inversetransformation process may be determined as illustrated in Table 2 belowby using “log2_min_transform_block_size_minus2” described above.

TABLE 2 if(min_transform_block_size_based_transform_skip_flag)  log2_skipTrafoSize_min      = max(2,log2_min_transform_block_size_minus2+2) else   log2_skipTrafoSize_min =2

That is, when min_transform_block_size_based_transform_skip_flag is 0,the size of a smallest transform block log2_skipTrafoSize_min referredto at the time of determining whether to skip the inverse transformationprocess is 2 (that is, indicates the block size of 4×4) and whenmin_transform_block_size_based_transform_skip_flag is 1, the size of asmallest transform block log2_skipTrafoSize_min has a larger value of 2and the value of log2_min_transform_block_size_minus2+2. In this case,when the size of a smallest transform block signaled by the encoder islarger than 4×4, the value of log2_skipTrafoSize_min becomeslog2_min_transform_block_size_minus2+2 and when the size of a smallesttransform block is the same as 4×4, the value of log2_skipTrafoSize_minbecomes 2.

Table 3 below schematically illustrates one example of a process ofdecoding the current transform block by determining whether to performinverse transform on the current transform block by referring to thesize of a smallest transform block log2_skipTrafoSize_min referred to atthe time of determining whether to skip the inverse transformationprocess as described above.

TABLE 3 Des- residual_coding( x0, y0, log2TrafoSize, cIdx ) { criptorif( transform_skip_enabled_flag && !cu_transquant_bypass_ flag    && (log2TrafoSize == log2_skipTrafoSize_min ) )      transform_skip_flag[ x0][ y0 ][ cIdx ] ae(v)     ... }

Referring to Table 3 above, when the current transform block sizelog2TrafoSize is the same as the size of a smallest transform blocklog2_skipTrafoSize_min referred to at the time of determining whether toskip the inverse transformation process, a flag transform_skip_flag thatindicates whether to perform the inverse transformation on the currenttransform block may be read. When the process is described in moredetail, in the case where the transform skipping is enabled (that is,transform_skip_enabled_flag =1) and the transformation and thequantization are not bypassed for each CU (that is,cu_transquant_bypass_flag=0), it is checked whether the currenttransform block size log2TrafoSize and the size of a smallest transformblock log2_skipTrafoSize_min referred to at the time of determiningwhether to skip the transformation process are the same as each other.Here, when the current transform block size log2TrafoSize is the same asthe size of a smallest transform block log2_skipTrafoSize_min referredto at the time of determining whether to skip the inverse transformationprocess, the decoder may read the flag transform_skip_flag thatindicates whether transform is performed on the current transform block.When a value of the flag transform_skip_flag is 1, the inversetransformation may be skipped. When the value of the flagtransform_skip_flag is 0, the inverse transformation may be performed.

In the aforementioned embodiments, methods have been described based onflowcharts as a series of steps or blocks, but the methods are notlimited to the order of the steps of the present invention and any stepmay occur in a step or an order different from or simultaneously as theaforementioned step or order. Further, it can be appreciated by thoseskilled in the art that steps shown in the flowcharts are not exclusiveand other steps may be included or one or more steps do not influencethe scope of the present invention and may be deleted.

It will be appreciated that various embodiments of the present inventionhave been described herein for purposes of illustration, and thatvarious modifications, changes, substitutions may be made by thoseskilled in the art without departing from the scope and spirit of thepresent invention. Accordingly, the various embodiments disclosed hereinare not intended to limit the technical spirit but describe with thetrue scope and spirit being indicated by the following claims. The scopeof the present invention may be interpreted by the appended claims andthe technical spirit in the equivalent range are intended to be embracedby the invention.

What is claimed is:
 1. A method of decoding an image, comprising:generating a prediction block for a current block; receiving informationof a block size, wherein the information of the block size is determinedby an encoder, and wherein whether a skip of a transform process isapplicable is determined based on a comparison of the information of theblock size with a current block size; decoding information indicatingwhether the skip of the transform process is applied, in response to thecomparison of the information of the block size with the current blocksize representing that the skip of the transform process is applicable;determining whether to perform an inverse transform on the current blockbased on the decoded information indicating whether the skip of thetransform process is applied; performing the inverse transform on thecurrent block to generate a residual block for the current block;reconstructing the current block based on the prediction block and theresidual block; and applying filtering on the reconstructed currentblock, wherein the prediction block is generated by performing intraprediction.
 2. The method of claim 1, wherein in response to the decodedinformation indicating whether the skip of the transform process isapplied indicating that the transform process is not performed, theinverse transform on the current block is skipped.
 3. The method ofclaim 1, wherein in response to the decoded information indicatingwhether the skip of the transform process is applied or not indicatingthat the transform process is performed, the inverse transform on thecurrent block is performed.
 4. The method of claim 1, wherein inresponse to a value of transform_skip_flag being 1, the transformprocess is not performed and is skipped on the current block, and inresponse to the value of transform_skip_flag being 0, the transformprocess is performed on the current block.
 5. The method of claim 4,wherein the information indicating whether the skip of the transformprocess is applied is a flag indicating whether to perform the inversetransform on the current block based on the information of the blocksize.
 6. The method of claim 1, further comprising performing an inversetransform on the current block according to a size of the current block,in response to a size of the current block satisfying a condition inwhich the skip of the transform process is not applied.
 7. The method ofclaim 6, the skip of the transform process is applied when the currentblock size is larger than 4×4 pixels.
 8. An apparatus to decode animage, comprising: at least a processor configured to: generate aprediction block for a current block; decode information of a block sizeand information indicating whether a skip of a transform process isapplied, wherein the information of the block size is determined by anencoder, and wherein whether the skip of the transform process isapplicable is determined based on a comparison of the information of theblock size with a current block size; determine whether to perform aninverse transform on the current block based on the decoded informationindicating whether the skip of the transform process is applied; performthe inverse transform on the current block to generate a residual blockfor the current block; reconstruct the current block based on theprediction block and the residual block; and apply filtering on thereconstructed current block, wherein the prediction block is generatedby performing intra prediction.
 9. The apparatus of claim 8, the skip ofthe transform process is applied when the current block size is largerthan 4×4 pixels.
 10. A method of encoding an image, comprising:generating a prediction block for a current block; generating a residualblock for the current block based on the prediction block; encoding theresidual block; decoding the encoded residual block; reconstructing thecurrent block based on the prediction block and the decoded residualblock; and apply filtering on the reconstructed current block, whereinthe prediction block is generated by performing intra prediction, andwherein the encoding the residual block comprises determininginformation of a block size, wherein whether a skip of a transformprocess is applicable is determined based on the information of theblock size compared with a current block size; determining whether toapply the skip of the transform process on the current block based on acomparison of the information of the block size with a current blocksize; and performing a transform on the current block to encode theresidual block.
 11. The method of claim 10, further comprising encodinginformation indicating whether the skip of the transform process isapplied to the current block.
 12. The method of claim 10, wherein inresponse to a value of transform_skip_flag being 1, the transformprocess is not performed and is skipped on the current block, and inresponse to the value of transform_skip_flag being 0, the transformprocess is performed on the current block.
 13. The method of claim 11,wherein in response to a determination that the skip of the transformprocess is applied to the current block, the information indicatingwhether the skip of the transform process is applied to the currentblock indicates that the transform on the current block is skipped. 14.The method of claim 13, further comprising performing the transform onthe current block according to a size of the current block, in responseto a size of the current block satisfying a condition in which the skipof the transform process is not applied.
 15. The method of claim 11,wherein in response to a determination that that the skip of thetransform process is not applied to the current block, the informationindicating whether the skip of the transform process is applied to thecurrent block indicates that the transform on the current block isperformed.
 16. The method of claim 12, wherein information indicatingwhether the skip of the transform process is applied is a flagindicating whether to perform the transform on the current block basedon the information of the block size.
 17. The method of claim 16,wherein the skip of the transform process is applied when the currentblock size is larger than 4×4 pixels.
 18. An apparatus to encode animage, comprising: at least a processor, configured to: generate aprediction block for a current block; generate a residual block for thecurrent block based on the prediction block; encode the residual block;decode the encoded residual block; reconstruct the current block basedon the prediction block and the decoded residual block; and applyfiltering on the reconstructed current block, wherein the predictionblock is generated by performing intra prediction, and wherein theencoding the residual block comprises performing a transform on thecurrent block based on information indicating whether a skip of atransform process is applied; and encoding information of a block sizeand the information indicating whether the skip of the transform processis applied, wherein whether the skip of the transform process isapplicable is determined based on a comparison of the information of theblock size with a current block size.
 19. The apparatus of claim 18,wherein the skip of the transform process is applied when the currentblock size is larger than 4×4 pixels.